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A quick disconnect, as it is used in manufacturing, provides a means of quickly disconnecting a line without the loss of hydraulic fluid or entrance of air into a system. The quick disconnect system is important for a number of reasons, not the least of which is that being able to disconnect quickly can save time, money, and in some cases, lives.
The quick disconnect consists of a coupling assembly with two halves, which are held together by a union nut. Each half contains a valve held open when the coupling is connected. When the coupling is open, it allows fluid to flow through in either direction. When it is disconnected, a spring in each half closes the valve, preventing the loss of fluid and the entrance of air. This happens quickly, shutting the system down almost immediately – hence, the name quick disconnect.
Quick disconnect couplings are designated as connecting devices designed to permit easy and immediate connection and separation of air or fluid for pneumatic or hydraulic lines without the need for tools. When a person has to stop and grab a tool in order to disconnect, it takes time and manpower. When installed in a fluid system, quick disconnect couplings save time by eliminating the need for system bleeding, recharging, or purging of air due to a lag time in the disconnection process.
The union nut has a quick-lead thread that allows the quick disconnect coupling to be connected or disconnected by turning the nut, which is the quickest and easiest way to shut it off. A quick disconnect is always triggered by hand rather than by machine. This makes it more reliable, since something unplanned could happen within the programming of a machine. Most manufacturers prefer the hands-on disconnect power.
The distance the nut must be turned varies with different styles of quick disconnect couplings. While one might require a quarter turn of the union nut, another might require a full turn. Some quick disconnect couplings require wrench tightening, while others are connected and disconnected by hand. Again, most companies prefer those controlled by hand rather than by wrench.
Some installations require the quick disconnect coupling to be safe tied with safety wire. This is a safety precaution, yet there are some couplings that do not require any form of safe tying. There are many individual differences, so every type of quick disconnect should be installed according to its respective instructions.
@horsebite - That is definitely true, but I wouldn't say that these fittings are particularly dangerous as long as they are used properly.
Just like anything else, they have a certain way they need to be handled, and if you do that they should do their job well with a minimum of risk.
Personally, I will definitely trade the need to be careful and use safety devices over the alternative of having to manually connect and disconnect hoses that don't have this kind of feature.
The requirement for safety wire where appropriate is a good one. Quick disconnect fittings are a great convenience, but they can be a common point of failure if you aren't really careful to make sure they are fully connected. Nothing like a face full of hot hydraulic fluid or a crack in the head from the metal end of the fitting to remind you that you didn't fully connect your hoses.